Add new gpu sink for DDLs

Having this as a sink rather than a Via allows us to do more aggressive things with threads and shared contexts.

Change-Id: I3ca1076686fa4f53387c12a9506e01910c1bc3e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/272016
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed